1. Field of the Invention
The present invention relates to lead frames and methods for producing the same, and in particular, the invention relates to lead frames for small semiconductor devices; having a large number of pins and to and an effective method for producing such frames.
2. Description of the Related Art
Lead frames used in resin mold type semiconductor devices are formed with a desired wiring pattern by stamping and etching metal sheets from sheet metal strips.
The density of semiconductor devices now in use has been greatly increased, and the size of the packages which include semiconductor devices has been greatly reduced. Accordingly, lead frames having inner and outer leads which are electrically connected to semiconductor devices must have a large number of pins.
It is well known that the width of the portions removed by stamping and etching is essentially the same as; the thickness of the metal sheet. Further, metal sheets which are too thin cannot be used, because this lowers the strength of the outer leads, and thus the number of pins provided by the lead frame for connection to a semiconductor device is limited.
Conventionally, lead frames prepared from metal sheets having a thickness of about 0.1 mm are limited to about 160 pins. Recently, however, a need has arisen for lead frames having more than 180 pins, and this need has been met by spacing the top ends of the inner leads from the semiconductor device, whereby it is possible to increase the number of pins or leads provided by the lead frame. This arrangement, however, results in a lengthening of the wire bonding distance between the semiconductor device and an the spaced inner leads, and thus problems arise with regard to wire bonding and the size of the semiconductor package is increased.